1. Field of the Invention
The present invention relates to a planar higher-harmonic high frequency oscillator for generating even-order harmonics of the fundamental wave such as a second harmonic, a fourth harmonic and the like through so-called push-push oscillation, and more particularly, to a multi-functional planar harmonic high frequency oscillator.
2. Description of the Related Art
An oscillator based on push-push oscillation is known as suitable for generating oscillation signals in a millimeter wave band and a microwave band. The oscillator based on push-push oscillation employs a pair of oscillation circuits which operate at the same fundamental frequency but at opposite phases to each other, and combines the outputs from these oscillation circuits to cancel out the fundamental wave components and extract even-order harmonic components to the outside. In such push-push oscillators, a planar harmonic oscillator, which has oscillation circuits each comprised of a resonant circuit made up of a planar circuit and an amplifier for oscillation, is used in a variety of applications because of its ability to generate output frequencies twice or more as high as fundamental wave f0 in a simple configuration. A planar harmonic oscillator using push-push oscillation is useful, for example, as an oscillation source for a high frequency network which operates, for example, in association with fiber-optic cables, or as an oscillation source for measuring devices. The present inventors have proposed, for example, in Japanese laid-open patent publication No. 2004-96693 (JP, P2004-96693A), a planar harmonic oscillator which is further reduced in size to facilitate its design and generates, for example, even-order harmonics of second harmonic wave 2f0 or higher with respect to fundamental wave f0.
FIG. 1A is a plan view illustrating the configuration of a conventional planar harmonic oscillator which employs injection synchronization for generating a frequency component twice as high as a fundamental wave, i.e., a second harmonic component, and FIG. 1B is a cross-sectional view taken along a line A—A in FIG. 1A.
The planar second-harmonic oscillator generally comprises a pair of amplifiers 1a, 1b for oscillation; micro-strip line 2 which serves as a high frequency transmission line within oscillation systems; and slot line 3 for coupling. Slot line 3 causes the two oscillation systems to oscillate in opposite phases to each other.
Micro-strip line 2 for oscillation is made up of signal line 2A routed on one principal surface of dielectric substrate 4, and ground conductor 5 formed substantially over the entirety of the other principal surface of dielectric substrate 4. Here, micro-strip line 2 is formed in a closed loop substantially in a rectangular shape.
The pair of amplifiers 1a, 1b for oscillation, each comprised of an FET (Field Effect Transistor) or the like, have their output terminals disposed on the one principal surface of dielectric substrate 4 to be opposite to each other, and are inserted in micro-strip line 2. In this way, micro-strip line 2 connects input terminals of the pair of amplifiers 1a, 1b for oscillation to each other, and the output terminals of the same to each other.
Slot line 3 is implemented by aperture line 3A formed in ground conductor 5 on the other principal surface of substrate 4, and is routed to vertically traverse two sections in central portions of micro-strip line 2 which is routed on the one principal surface of substrate 4. Slot line 3 extends in the vertical direction by λ/4 respectively from the sections of micro-strip line 2 which are traversed by slot line 3, where λ represents the wavelength corresponding to an oscillation frequency (i.e., fundamental wave f0), later described. Output line 6, superimposed on slot line 3, is connected to the center of a portion of micro-strip line 2 (the upper side in the figure) which connects the outputs of the pair of amplifiers 1a, 1b for oscillation. Output line 6 is routed on the one principal surface of substrate 4.
In the foregoing oscillator, micro-strip line 2 is electromagnetically coupled to slot line 3 to form two oscillation systems, as shown in the left and right halves of the figures. In this configuration, a high frequency signal in an unbalanced propagation mode, which propagates through micro-strip line 2, is converted into a balanced propagation mode of slot line 3. Since the balanced propagation mode of slot line 3 involves a propagation which has opposite phases at both sides of aperture line 3A, eventually causing the two oscillation systems to oscillate in phases opposites to each other. Since the oscillation frequency (fundamental wave f0) in the oscillation systems generally depends on the length of each oscillation closed loop, the oscillation systems are designed such that the respective oscillation systems oscillate at the same oscillation frequency.
In the configuration as described above, at the midpoint of micro-strip line 2 which connects the outputs of the pair of amplifiers 1a, 1b for oscillation to each other, the fundamental wave (f0) component and odd-order harmonic components in the oscillation frequencies are in opposite phases to each other to provide null potential. On the other hand, even-order harmonics of a second harmonic wave or higher are combined for delivery. However, since harmonics of fourth order and more have relatively low levels as compared with the second harmonic, the fundamental wave (f0) component and other higher-harmonics are suppressed to provide second harmonic wave 2f0 on output line 6. Here, if second harmonic 2f0 is also suppressed, fourth harmonic wave 4f0, which has the next highest level, can be extracted through output line 6.
Since slot line 3 is extended by a quarter wavelength relative to fundamental wave f0 from the upper and lower sections of micro-strip line 2, the respective ends of slot line 3 are electrically open ends, viewed from the positions at which slot line 3 traverses micro-strip line 2. Therefore, the oscillation component of fundamental wave f0 is efficiently transmitted to a negative feedback loop through slot line 3, thus increasing the Q value of the oscillator circuit. The length λ/4, by which slot line 3 is extended, need not be strictly equal to λ/4 because this may be such a length that permits the ends of slot line 3 to be regarded as electrically open ends.
However, the planar second-harmonic oscillator in the foregoing configuration simply increases the oscillation frequency, but is not designed in full consideration of providing a multi-functional planar oscillator. Specifically, when the aforementioned planar second-harmonic oscillator is used to design, for example, an FSK (frequency shift keying) circuit, only way available therefor is to provide two sets of planar second-harmonic oscillators having two different oscillation outputs f1, f2 which are switched from one to the other with an electronic switch. Also, even when an ASK (amplitude shift keying) circuit is designed, outputs of oscillators are generally turned on/off by an electronic switch in a similar manner. Any of these circuits requires an electronic switch or the like for switching or turning on/off the outputs, and the electronic switches are disposed separately from a planar oscillator, thus resulting in complicated circuit designs.